A known vehicle steering device is provided with a so-called rack and pinion type steering gear mechanism including a rack bar. The rack and pinion type steering gear mechanism is connected to a pair of knuckle arms for respective right and left vehicle steered wheels via a steering link mechanism so as to steer the steered wheels. Especially, when the steering gear mechanism having the rack bar is used for an electrically powered steering device, the rack bar is integrally provided with a rack teeth portion engaged with a pinion teeth portion provided at an end of a steering shaft connected to a steered wheel, a ball screw portion for the electrically powered steering device, and an end push guiding portion.
According to the steering gear mechanism with the above-described structure, a surface hardening heat treatment such as a quenching treatment is applied to the rack bar so as to assure surface hardness demanded for predetermined portions thereof. More particularly, a resistance quenching treatment is applied to the rack teeth portion so as to assure surface hardness for the teeth portion. The backside of the rack teeth portion, on which teeth is not provided, is not applied with the surface hardening heat treatment. The ball screw portion is applied with an induction quenching treatment so as to assure surface hardness on an entire surface area thereof. The end push guiding portion, which is not demanded with the surface hardening heat treatment, remains unmachined. Therefore, each portion of the known rack bar can be assured with the surface hardness required for each.
However, according to the above-described rack bar and the method of heat-treating the rack bar, respective quenching devices and methods are required independently for the respective portions so as to assure the surface hardness respectively required. This may cause decrease of productivity of the rack bar.
Further, when the teeth of the rack teeth portion is applied with the quenching treatment, the rack teeth portion may be undesirably twisted or warped. In this case, special attentions, special measure, or special quality controls may be required for avoiding the twist or warp of the rack teeth portion. This may also cause the decrease of productivity of the rack bar.
The present invention therefore seeks to provide an improved method of heat-treating a rack bar, which enables to assure surface hardness required for each portion, and which enables to improve manufacturing precision for manufacturing the rack bar, thereby leading to widely enhancing the productivity of the rack bar. Further, the present invention therefore seeks to provide an improved rack bar which can possess portions with required surface hardness, and can be manufactured with an improved manufacturing precision and productivity.